Ind. Eng. Chem. Res. 2002, 41, 5321
5321
REVIEWS Carbonyl Sulfide: A Review of Its Chemistry and Properties Paris D. N. Svoronos† and Thomas J. Bruno*,‡ Department of Chemistry, Queensborough College of the City University of New York, Bayside, New York 11364, and Physical and Chemical Properties Division, National Institute of Standards and Technology, Boulder, Colorado 80303
A number of years have elapsed since the last comprehensive review of the chemical properties of carbonyl sulfide (COS) was presented in 1957. Since that time, some important new issues have arisen regarding this fluid. The presence of COS in industrial product streams has always been an important consideration for chemical engineers. An example of a relatively new industrial issue is the presence of naturally occurring COS in liquefied petroleum gas (LPG). It is believed that the hydrolysis of this COS is the cause of corrosion and compliance-testing failures in the LPG industry. New applications of COS have arisen in recent years, such as its use as an agricultural fumigant. Environmental issues also have become more of a concern recently. These issues, as well as many other chemical and physical property issues in science and industry, make this an appropriate time to revisit the chemistry of this interesting fluid, paying special attention to recent observations. In this review, we treat the chemical preparation of COS, its major physical properties, and its major chemical reactions, and then we discuss engineering consequences, applications, and environmental issues. Contents 1. Introduction 2. Preparation of Carbonyl Sulfide 3. Physical Properties of Carbonyl Sulfide 4. Health and Safety Issues Pertaining to Carbonyl Sulfide 5. COS Chemistry The Claus Reaction Hydrolysis of Carbonyl Sulfide Alumina-Catalyzed Hydrolysis Impregnation of Cations in Alumina and Titania Use of Catalysts Other Than Alumina Kinetics of Alumina-Catalyzed Hydrolysis Hydrolysis in the Presence of Tertiary Alkanolamine Solutions 6. Effect of COS in Bulk Hydrocarbons 7. Environmental Considerations Atmospheric COS Marine COS COS in Plants Anthropomorphic COS 8. Recent Application: COS as an Agricultural Fumigant 9. Summary, Conclusions, and Future Directions 10.1021/ie020365n
1. Introduction 5321 5321 5322 5322 5323 5323 5324 5324 5324 5325 5325 5326 5326 5327 5327 5328 5328 5329 5329 5330
Carbonyl sulfide (COS or SdCdO, IMIS R220, CAS 463-58-1), also known as carbon oxide sulfide, carbon monoxide monosulfide, or carbon oxysulfide, is a fluid that is present in minor amounts in petroleum and coal refinery gases1 and, to a greater extent, in coal gasification streams. The chemistry and properties of this material were covered by Ferm2 in 1957, in a review that focused on its preparation, properties and chemical reactions. Recent industrial applications and environmental problems associated with COS require that it be given renewed attention. 2. Preparation of Carbonyl Sulfide The history of preparation of carbonyl sulfide was documented in Ferm’s earlier review.2 The best laboratory or plant synthetic procedure involves the reaction of a potassium thiocyanate (KCNS) solution with 50% (mass/mass) sulfuric acid (H2SO4) at 30 °C according to eq 1
KCNS + 2H2SO4 + H2O f KHSO4 + NH4HSO4 + COS (1) The subsequent purification step involves the removal of carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen sulfide (H2S), carbon difulfide (CS2), formaldehyde (HCOH), and hydrogen cyanide (HCN), which are * To whom correspondence should be addressed, bruno@ boulder.nist.gov. † Queensborough College of the City University of New York. ‡ National Institute of Standards and Technology.
This article not subject to U.S. Copyright. Published 2002 by the American Chemical Society Published on Web 10/04/2002
5322
Ind. Eng. Chem. Res., Vol. 41, No. 22, 2002
formed as side products (these are not shown in eq 1 for the sake of clarity). To accomplish this purification, the product gas is first treated with a mixture of saturated aqueous cupric sulfate (CuSO4) solution and concentrated H2SO4 (50% vol/vol ratio) to remove hydrogen sulfide (H2S). The gas is subsequently passed through an aqueous potassium hydroxide (KOH) solution to remove all remaining acid gases. An extraction with a 25% (mass/mass) solution of aniline in ethanol eliminates carbon disulfide (CS2), and finally, further treatment with concentrated H2SO4 yields anhydrous COS.3 Commercial-grade carbonyl sulfide (usually sold with a stated purity of 97.5%) can be degassed while frozen in liquid nitrogen (77 K, -196 °C), and if needed in solution, it can be mixed with helium before introduction into a downstream process or reaction vessel.4 Alternatively, a purity of 96% COS can be obtained by passing the fluid through a 30% (mass/mass) aqueous KOH solution to remove carbon dioxide and hydrogen sulfide. The recovered fluid is subsequently stored in a sampling balloon or other suitable vessel at atmospheric pressure.5 Often, one observes surprising quantities of hydrogen sulfide in commercial sources of COS, even sources that are listed as 97.5% pure. On a small scale, this H2S can be removed by passing the fluid through a bed of 4A molecular sieves. Carbonyl sulfide is also found naturally in petroleum fractions, making it an issue in numerous petrochemical processes. Because propane and carbonyl sulfide have similar boiling temperatures (-44.5 °C for propane, -50.2 °C for COS), after separation, 90% of the petrochemical COS is usually found in the propane fraction. Only 10% is found in the ethane fraction, while virtually none occurs in the butane fraction. Carbonyl sulfide also occurs in natural gas, but because natural gas is usually saturated with water at the well head (before processing), much of this COS is hydrolyzed to hydrogen sulfide before processing. Recently, however, the frequency of COS being identified in downstream extended natural gas analyses has been on the rise. Quantitative trapping of COS is feasible through a cryogenic trapping procedure that functions without liquid cryogens at -150 °C.6 Carbonyl sulfide is used in the industrial synthesis of thio acids, sulfur-trisubstituted carbinols, substituted thiazoles (found in flavors and fragrances and used as ingredients in pharmaceuticals such as anti-inflamatories), and substituted thiocarbamic acids. High yields are obtained, particularly in the case of substituted thiazoles. 3. Physical Properties of Carbonyl Sulfide Carbonyl sulfide is a colorless gas (at ambient temperature and pressure) that has an unpleasant rotten egg odor, except when it is very pure. It is somewhat soluble in water (0.80 mL/mL of water at 13.5 °C, 1 atm) and in toluene (15.0 mL/mL of toluene at 22.0 °C, 1 atm). It is very soluble in potassium hydroxide solutions, in which its hydrolysis to H2S is catalyzed. In water, COS slowly decomposes by hydrolysis into hydrogen sulfide. It is shipped in steel cylinders as a liquefied gas under its own vapor pressure of 1.10 MPa (160 psig) at 21 °C.7 Carbonyl sulfide is compatible with many metals such as aluminum, copper, Monel, carbon steel, 300-series stainless steels, and brass. The compatibility is considerably reduced in the presence of moisture, as is
commonly observed with many acid gases. Synthetic polymers such as PVC and Teflon are very compatible with COS. The major physical properties of carbonyl sulfide, as well as the structurally similar compounds carbon dioxide and carbon disulfide, are provided in Table 1. The properties of propane, butane, and hydrogen sulfide, often found naturally with carbonyl sulfide, also appear in Table 1. 4. Health and Safety Issues Pertaining to Carbonyl Sulfide Carbonyl sulfide is classified as an irritant to skin, eyes, nose, throat, and lungs, causing cough and sneezing, probably because of its ability to undergo hydrolysis and form hydrogen sulfide.8 In low to moderate vapor concentrations, the irritant action appears to be most noticeable. This irritation can take the form of severe and painful conjunctivitis, increased production of tears (lachrymation), and increased sensitivity to light (photophobia).9 Low to moderate concentrations can also cause nausea, diarrhea, profuse salivation, headache, and mental confusion. The term “low to moderate concentration” in this context can vary from person to person; thus, it is impossible (and inadvisable) to fix more precise limits on the concentration. Rather, the seriousness and extent of the symptoms observed are the important factors. Exposure to high concentrations in the vapor phase can cause sudden collapse, tremors, blurred vision, and tachycardia. Prolonged exposure leads to unconsciousness, coma, and death, which are attributed to respiratory paralysis.9 Slow recovery, slowed pulse, cardiac dilation, and possible amnesia usually follow sublethal exposure to high concentrations. Recovery is usually complete in nonfatal cases. As before, the term “high concentration” of vapor cannot and should not be precisely defined,7,10 but concentrations of 0.1% (vol/vol) and higher can produce death in 2 h or less.8 Chronic exposure of 50 ppm (mass/mass) carbonyl sulfide (to noncholesterol fed rabbits) of between 0.5 and 12 weeks showed no histotoxic effect on the intimal or subintimal morphology of coronary arteries or the aorta.8 Similar studies of 50 ppm (mass/mass) carbonyl sulfide exposure to rabbits for 7 weeks showed no effect on the myocardial ultrastructure,11,12 and only a slight elevation of the mean serum or aortic cholesterol concentration was observed.13 Bhatia14 reports that carbonyl sulfide concentrations greater than 20 mg/L can cause irritation in the upper human respiratory system. Houriet and Louvier15 classify carbonyl sulfide as a substance that belongs in toxicity class 2 with a threshold toxicity value of 23 ppm (mass/mass) for mammals. As of 2002, however, the U.S. Environmental Protection Agency (EPA) has neither established a reference dose for chronic oral exposure (RfD) or a reference concentration for chronic inhalation exposure (RfC) for the compound nor classified it with respect to its potential carcinogenicity.16 During the seventh International Working Conference on Stored-Product Production in Beijing, China, in 1998, Chinese scientists reported the results on a toxicity test of carbonyl sulfide as applied to rats. Carbonyl sulfide was classified as a low-toxicity fumigant with no apparent irritation to eye or derma. It had no mutagenesis effect, and the LC50 was determined to be in excess of 2000 mg/m3. All subchronic, teratology, generation, chronic, and carcinogenesis tests showed a maximal
Ind. Eng. Chem. Res., Vol. 41, No. 22, 2002 5323 Table 1. Physical Properties of Carbonyl Sulfide (COS), Structurally Similar Compounds, and Compounds That Occur Naturally with Carbonyl Sulfidea fluid physical property
COS
CO2
SO2
CS2
H 2S
C2H6
C3H8
relative molecular mass (kg/kmol) critical temperature (K) critical pressure (Pa) critical volume (m3/kmol) critical compressibility factor melting point (K) triple-point temperature (K) triple-point pressure (Pa) normal boiling point (K) liquid molar volume (m3/kmol) ideal-gas heat of formation (J/kmol) ideal-gas Gibbs heat of formation (J/kmol) ideal-gas absolute entropy (J/kmol‚K) standard absolute entropy (J/kmol‚K) standard heat of formation (J/kmol) standard Gibbs heat of formation (J/kmol) enthalpy of fusion at mp (J/kmol) heat of combustion (J/kmol) acentric factor radius of gyration (m) solubility parameter (J/m3)1/2 dipole moment (c‚m) van der Waals volume (m3/kmol) van der Waals area (m2/kmol) refractive index flash point (K) upper flammability limit (vol % in air) lower flammability limit (vol % in air) upper flammability temperature (K) lower flammability temperature (K) autoignition temperature (K) gas density at 0 °C (g/L) liquid density (g/cm3) at T (K) Cp (J/g‚°C)b Cv (J/g‚°C)b
60.0764 378.8 6.35 × 106 0.1351 0.272 134.35 134.3 59.3211 223 0.050 98 -1.42 × 108 -1.69 × 108 2.31 × 105 2.31 × 105 -1.42 × 108 -1.69 × 108 4.73 × 106 -5.48 × 108 0.097 1.27 × 10-10 1.81 × 104 2.37 × 10-30 0.0259 4.05 × 108 1.3785 NA 29 12 199 186 NA NA 1.274 174 0.6908 NA
44.0098 304.21 7.38 × 106 0.0094 0.274 216.58 216.58 5.19 × 105 NA 0.037 27 -3.94 × 108 -3.94 × 108 2.14 × 105 2.14 × 105 -3.94 × 108 -3.94 × 108 9.02 × 106 0.00 × 100 0.224 1.04 × 10-10 1.46 × 104 0 0.197 3.23 × 108 1.000 41 NA NA NA NA NA NA 1.977 NA 0.8577 0.6548
64.0648 430.75 7.88 × 106 0.122 0.269 200 197.67 1674.39 263.13 0.043 82 -2.97 × 108 -3.00 × 108 2.48 × 105 2.48 × 105 -2.97 × 108 -3.00 × 108 7.40 × 106 0.00 × 100 0.245 1.66 × 10-10 1.23 × 104 5.44 × 10-30 0.025 73 4.03 × 108 1.357 NA NA NA NA NA NA 2.927 1.455 263 NA NA
76.143 552 7.90 × 106 0.160 0.275 161.58 161.11 1.4944 319.375 0.060 64 1.17 × 108 6.68 × 107 2.38 × 105 1.51 × 105 8.90 × 107 6.47 × 107 4.39 × 106 -1.08 × 109 0.111 1.57 × 10-10 2.04 × 104 0 0.312 4.75 × 108 1.624 09 243.15 50 1.3 300 228 363.15 1.627 1.293 273 NA NA
34.0819 373.53 8.96 × 106 0.0985 0.284 187.68 187.68 2.32 × 104 212.8 0.035 86 -2.06 × 107 -3.34 × 107 2.06 × 105 2.06 × 105 -2.06 × 107 -3.34 × 107 2.38 × 106 -5.18 × 108 0.094 6.38 × 10-11 1.80 × 104 3.23 × 10-30 0.018 72 3.10 × 108 1.005 85 NA 45.5 4.3 199
30.0696 305.32 4.87 × 106 0.1455 0.279 90.352 90.352 1.13 184.55 0.055 23 -8.38 × 107 -3.19 × 107 2.29 × 105 2.29 × 105 -8.38 × 107 -3.19 × 107 2.86 × 106 -1.43 × 109 0.099 1.83 × 10-10 1.24 × 104 0 0.2734 4.24 × 108 1.184 89 NA 13 2.9 154 137 745 NA 0.548 183 1.7142 0.143 76
44.0965 369.83 4.25 × 106 0.200 0.276 85.47 85.47 1.69 × 10-4 231.11 0.075 70 -1.05 × 108 -2.44 × 107 2.70 × 105 2.70 × 105 -1.05 × 108 -2.44 × 107 3.52 × 106 -2.04 × 109 0.152 2.43 × 10-10 1.31 × 104 0 0.037 57 5.59 × 108 1.286 14 NA 9.5 2 189 169 723 1.97 0.582 231 1.6255 1.4368
533.15 1.5392 0.993 214 1.0042 0.7573
a Rowley, R. L.; Wilding, W. V.; Oscarson, J. L.; Zundel, N. A.; Marshall, T. L.; Daubert, T. E.; Danner, R. P. DIPPR Data Compilation of Pure Compound Properties; Design Institute for Physical Properties, AIChE, New York, 2001. b Gas at 25 °C, 1 atm.
effectless level (listed as NML in the Chinese literature) of at least 20 g/m3, which allows COS to be clasified as a low-toxicity and safe fumigant.17 Carbonyl sulfide is a flammable compound with an NFPA Hazard Rating of 4, and its fires can restart after being extinguished. It is often recommended that, unless a leak of carbonyl sulfide gas can be stopped quickly, small fires should be allowed to burn themselves out. Moderately sized fires can be fought with foam or water. Massive cargo fires should be fought with unmanned hoses or should be allowed to burn. It also must be remembered that the fluid is relatively soluble in water, with concentrations of more than 1000 mg per liter of water. Fires extinguished with water can therefore result in water contaminated with hydrogen sulfide and carbonyl sulfide.10 This effect can be especially pronounced if a caustic material such as potassium hydroxide is also present, because these materials have a strong catalytic effect on the hydrolysis of COS to hydrogen sulfide.
5. COS Chemistry The Claus Reaction The awareness of the importance of carbonyl sulfide in industry has recently been on the rise as a result of environmental concerns. For instance, in the Claus reaction, sulfur is industrially prepared by the hightemperature oxidation of hydrogen sulfide according to
the sequence of eqs 2 and 3, which leads to net eq 4
3H2S + 3/2O2 f 2H2S + SO2 + H2O
(2)
2H2S + SO2 f 3S + 2H2O
(3)
3H2S + 3/2O2 f 3S + 3H2O
(4)
Thus, first, the combustion of hydrogen sulfide leads to the formation of sulfur dioxide and water (eq 2). The formed sulfur dioxide reacts with more hydrogen sulfide to yield water and condensed sulfur (eq 3). However, the combustion process also involves the formation of carbon dioxide, which reacts with hydrogen sulfide to yield mainly carbonyl sulfide, as well as carbon disulfide, according to eqs 5 and 6
CO2 + H2S f COS + H2O
(5)
CO2 + 2H2S f CS2 + 2H2O
(6)
The formed carbonyl sulfide is a polluting byproduct in the Claus reaction because it undergoes hydrolysis to yield carbon dioxide and hydrogen sulfide18 according to the equation
COS + H2O f CO2 + H2S
(7)
As a result, additional costly treatments must be employed to remove the carbonyl sulfide downstream from the Claus converter. These include the Beavon
5324
Ind. Eng. Chem. Res., Vol. 41, No. 22, 2002
Table 2. Rate Constant (k298) and Activation Energy (Eact) Values for the Hydration Reactions of Carbon Dioxide, Carbonyl Sulfide, and Carbon Disulfide compound
pH
CO2
acid base
0.03 s-1 104 M-1 s-1
k298
Eact (kcal) 16-19 5-13
35-38 35-38
COS
acid base
2 × 10-5 s-1 13 M-1 s-1
20-23 12-14
18, 23 18, 23
CS2
acid base
,10-5 s-1 10-4-10-3 M-1 s-1 1-10 M-1 s-1
ref
20, 41 20, 41, 43 41
sulfur removal process19 and the SCOT process,20 in which catalytic reduction with hydrogen or reducing gas mixtures convert all sulfur content to hydrogen sulfide. In general, the hydrolysis products, carbon dioxide and hydrogen sulfide, are much easier to remove from the refinery gas streams than carbonyl sulfide.21 Hydrolysis of Carbonyl Sulfide The hydrolysis of carbonyl sulfide has been studied since the 1950s as a secondary reaction in viscose ripening22,23 (a step in the manufacture of Rayon from natural cellulose) and for the removal of sulfide in industrial products and waste gas.24,25 Carbonyl sulfide has been identified as the only source of sulfides [including the sulfide and hydrogen sulfide (HS-) anions as well as hydrogen sulfide] in open waters.26,27 Further research has led to the investigation and detection of ocean sulfide by G. A. Cutter and co-workers,28-30 as well as to the viability of the HS- ion to serve as a new sea ligand.27,31 The hydrolysis of carbonyl sulfide is used as a test for Bronsted basicity, because the first step of the base interaction involves the nucleophilic attack of the basic hydroxyl groups on the COS carbon atom.32-34 The process has been studied over a widely buffered range (of pH 4-10).18 The reaction has two rate-determining steps that are distinguishable at the extreme parts of the pH range. The basic-medium (high-pH) reaction produces monothiocarbonate (HCO2S-), whereas the acid pathway is unimolecular for pH 4-6. Monothiocarbonate appears to be the hydrolysis intermediate under conditions applicable to most natural waters. Calculated steady-state thiocarbonate concentrations are several orders of magnitude less than those of carbonyl sulfide.35-44 Table 2 lists the rate constant and energy of activation values for the hydration reactions of carbon dioxide, carbonyl sulfide, and carbon disulfide. The hydrolysis has been observed to be pseudo-firstorder, with k ) 1.04 × 1011e-10 000/T s-1. Increasing the hydroxyl concentration leads to a bimolecular rate constant k ) 8.12 × 109e-60 400/T M-1‚s-1 and a lifetime for dissolved COS of 1.7 days. The bimolecular rate constant does not incorporate the pseudo-first-order approximation and takes explicit account of both reactant species concentrations. A two-layer diffusion model with accurate Henry’s law constants and hydrolysis rate constants can determine the air-sea exchange flux. The lifetime for the transport of carbonyl sulfide from the troposphere to the ocean with respect to this flux is about 12 years. It appears that the ocean is a good source of COS despite the fact that hydrolysis occurs.45,46 Alumina-Catalyzed Hydrolysis The poisoning effect of sulfur on industrial heterogeneous catalysts has been established.47 Bimetallic reforming catalysts have been seriously affected by sulfur
quantities as low as a few parts per million (mass/mass). For instance, the activity of the Fe-Cu-K catalyst surface used in the Fischer-Tropsch synthesis is reduced by 50% by 4 mg of sulfur per gram of catalyst.48 The refinery process reactors are seriously corroded by feedstock sulfur.49 It is therefore not surprising that much of the catalytic work on COS chemistry has been geared toward defining catalytic and fouling mechanisms. Early studies of carbonyl sulfide hydrolysis mainly dealt with treatment of the Claus tail gas above 200 °C to meet the requirements of environmental laws.50-55 Eliminating COS from the gaseous effluent appeared to improve the Claus process while at the same time protecting the environment.56 Recent studies have also examined the desulfurization of coke oven gas (COG), which is used as a chemical feedstock and as a fuel gas similar to city gas.57 The removal of all sulfur compounds by converting them to hydrogen sulfide has been the standard procedure.58 Catalytic studies on the kinetics of carbonyl sulfide hydrolysis have involved metal oxides such as γ-alumina in lower temperature ranges.33,59,60 It was noted that the time required for the deactivation of the alumina was only 2 h. Moreover, the catalytic activity was dramatically reduced as the reaction temperature and oxygen content increased. On the other hand, the rate appeared to increase with increasing catalyst basicity.61 Alumina and other commercial equilibrium-shift catalysts have been used downstream of Claus burners to convert carbon monoxide to carbon dioxide before it reacts with hydrogen sulfide to yield carbonyl sulfide.62-64 It appears63 that these catalysts preferentially catalyze the water-gas shift reaction during the rapid cooling that occurs downstream of the combustion reactions. As a result, the prevention of carbonyl sulfide formation might be an effective alternative to the conventional treatments, which focus on the removal of the fluid after formation. Shift catalysts proposed by Gens and his group64 remove up to 99% of the sulfur compounds in a modified Claus converter but do not have the potential to convert carbonyl sulfide. FT-IR and mass spectrometric analyses using a fixedbed stainless steel reactor indicated the formation of COS after catalytic oxidation of carbon disulfide over atmospheric particles. CaO showed the strongest catalytic activity for oxidizing carbon disulfide, followed by Fe2O3 and Al2O3. SiO2 displayed the weakest catalytic activity. The collected atmospheric particles consisted of Ca(Al2Si2O8)‚4H2O, the major component of concrete cement. The concentration of adsorbed oxygen gas on the catalytic surfaces enhanced the formation of carbonyl sulfide.65 Impregnation of Cations in Alumina and Titania An alumina catalyst impregnated with potassium carbonate was found to have a high catalytic activity57 with carbonyl sulfide hydrolyzed in the low-temperature range of 10-80 °C. The reaction kinetics appears to be first-order with respect to COS and zeroth-order with respect to water. However, once the water vapor pressure increased in the region of 1.5 kPa at 40 °C, the rate decreased rapidly because of a large amount of water that became adsorbed on the catalyst. Water adsorption on catalytic surfaces has emerged as a recurring theme in work on catalyzed COS hydrolysis.
Ind. Eng. Chem. Res., Vol. 41, No. 22, 2002 5325
Small amounts (less than 1000 ppm, mass/mass) of sodium ions, such as in the form of oxide, Na2O, appear to decrease the number of acid centers on commercial alumina,66-68 thus reducing the activity toward COS hydrolysis. The crystallinity of alumina increases the Lewis acidity because of the presence of a higher number of aluminum cations in the tetrahedral coordination in the crystallized samples.69 The infusion of larger quantities (more than 1000 ppm, mass/mass) of sodium oxide increases the basicity of the catalyst by preventing sodium migration into the bulk, thus producing a larger number of surface hydroxyl anions.70 Further studies using the same γ-alumina catalyst doped with alkali metal oxides (Na2O, K2O, Cs2O) and alkali earth metal oxides (MgO, CaO, BaO) in the temperature range 45-100 °C were conducted by S. Tan et al.61 The correlation between the catalytic activity and the content of alkali oxides was found to be different from that of the alkali earth metal oxides. A direct linear relationship was found to exist between the catalytic activity and the content of alkali metal oxides when the content is relatively low. In general, the metal oxide content showed peak activity at about 5 mol %. Moreover, a compensation effect was observed during the study of hydrolysis reaction rate constants at various temperatures that was linearly related to the metal’s ionic radius. The compensation effect, which was reduced to a proportionality factor, described the simultaneous increase of the activation energy and the preexponential factor in the temperature-dependent description of the hydrolysis rate constant. The alumina-catalyzed hydrolysis of COS was kinetically monitored in the temperature range 30-140 °C. The catalyst was impregnated to incipient wetness (barely perceptible wetness) with aqueous solutions of metal salts and then dried at 120 °C and calcinated at 550 °C for 2 h each. Among all catalysts used, K2OPt/Al2O3 appeared to exhibit better stability and higher activity than K2O/Al2O3 or pure Al2O3. All reactions appeared to follow first-order kinetics with activation energy values of 57.80 kJ/mol using pure Al2O3, 52.50 kJ/mol for K2O/Al2O3, and 44.71 kJ/mol for K2O-Pt/ Al2O3. The hydrolysis of COS was achieved at lower temperature using K2O-Pt/Al2O3, in contrast to CS2, which is hydrolyzed at higher temperatures.4 Use of Catalysts Other Than Alumina The study of the effect of catalytic surfaces other than alumina has involved TiO2,32,34,70-72 ZrO2,32,34,71,72 and ZnO.72 The relative activity toward carbonyl sulfide hydrolysis follows the sequence ZrO2 . Al2O3 > TiO2, with ZnO being essentially inert, with the kinetics being the same as those for the uncatalyzed measurement.73 The activity was monitored by Fourier transform infrared (FTIR) spectroscopy (with the guidance of quantum chemistry calculations), which confirmed the rate proportionality with respect to the amount of carbon dioxide adsorbed on the surfaces in the form of hydrogen carbonate. In the case of ZnO, both FTIR spectroscopy and application of the semiempirical GEOMOS-GREEN calculation method indicated the formation of the monothiocarbonate adduct rather than the hydrogen thiocarbonate anion.72,73 The full d shell of the d10 configuration of Zn appears to contribute to the decreased basicity of the hydroxide bound on the ZnO surface when compared to other oxides such as TiO2 and ZrO2 that have an incomplete d shell.
FTIR spectroscopy was also used to show that the mechanism on titania appears to be similar to that of alumina at low temperature.74 The workers monitored the peaks at 2250 cm-1 (S-H stretch) and 1550 and 1320 cm-1 (CO2)72 and concluded that the formation of sulfate on the titania surface increases the surface acidity and therefore decreases the rate of hydrolysis. The calculated activation energy governing hydrolysis by the various oxide catalysts follows the sequence titania > alumina > zirconium, whether the nucleophilic attack is by the hydroxyl anion (42.3, 29.4, and 17.8 kcal/mol, respectively) or by water (54.1, 37.1, and 28.0 kcal/mol, respectively).72 At low temperature, the rate appears to be enhanced once the sulfate is removed by hydrogen sulfide (eq 8) without reducing Ti4+ to Ti3+
SO42- + H2S f S + SO2 + H2O
(8)
Generally, the sulfated titania surface is reduced at appreciably lower temperature, 400 °C (673 K), for 60 min74 than the alumina surface. The sulfate reduction occurs on titania at 450 °C (723 K) when hydrogen gas is employed, whereas the same process on alumina occurs at an even higher temperature.75 Because the Claus operating temperatures are in the range of 250350 °C (523-623 K), hydrolysis of COS and formation of hydrogen sulfide regenerates the titania catalyst to a much greater extent than in the case of the alumina catalyst.76 Kinetics of Alumina-Catalyzed Hydrolysis Low-temperature (30-250 °C) alumina catalysts of different surface areas with low COS concentrations (150 ppm, mass/mass) exhibit Langmuir-Hinshelwood kinetics. High-temperature experiments (250 °C, 523 K) indicate that the surface hydrolysis of thiocarbonate is the rate-determining step.76-78 Data obtained at lower temperatures (30-60 °C, 303-333 K) indicate that the rate of COS hydrolysis decreases monotonically with increasing water concentration. The high-temperature data are consistent with the Langmuir-Hinshelwood kinetics where the products are not adsorbed. The COS adsorption (or the surface reaction of adsorbed COS) and the water-derived intermediate is the rate-determining step. A correlation can be drawn on the importance of the surface area of the alumina catalyst as a design parameter.77 A laboratory Bert reactor was used to calculate the reaction rates for carbonyl sulfide hydrolysis at 270330 °C on both alumina (Kaiser 201) and titania (CRS 31) catalysts with water concentrations in the 2-12% (mole/mole) range and carbonyl sulfide concentrations in the 0.5-2.0% (mole/mole) range.71 The best-fitting rate function corresponded to the Eley-Rideal model, even though the catalytic activities were not the same. Titania appeared to be the more efficient catalyst primarily because of the greater ability of water vapor to adsorb on Kaiser 201 (even at 500 °C) as expressed by the smaller value of K3 seen in Table 3. Adsorption of water inhibits the hydrolysis abilities of both catalysts, especially in the case of alumina, despite its slightly lower activation energy (Eact) for the carbonyl sulfide hydrolysis. However, the relatively more favorable adsorption of water on the alumina surface appears to block the access of COS molecules to the active basic sites. It is safe to assume that the relatively large partial
5326
Ind. Eng. Chem. Res., Vol. 41, No. 22, 2002
Table 3. Regression Parameters for the Eley-Rideal Modela for the Catalytic Hydrolysis of Carbonyl Sulfide54 catalyst regression parameter
titania (CRS31)
alumina (Kaiser 201)
kb K3c Eactd He
exp[4.45 - (5.024 × 103)/T] exp[-13.09 + (8.101 × 103)/T] 41.77 67.35
exp[0.835 - (3.039 × 103)/T] exp[-15.89 + (1.001 × 104)/T] 25.27 83.22
a Regression parameters are applicable to the best model function in the following equation over the range 270-330 °C: -rcos) k1K3PcosHwater/(1+ K3Pwater). b Parameter array at the first iteration (mol/g‚h‚kPa). c Parameter of the rate function (l/kPa). d Activation energy for the irreversible hydrolysis reaction (kJ/ mol). 5Heat of adsorption for water on the catalyst (kJ/mol).
pressure of water vapor (about 100 kPa) in industrial Claus plants substantially affects the rate-retarding influence of adsorbed water.70,77 The ability of titania to hydrolyze carbonyl sulfide was estimated to exceed 90% by simulation of a Claus converter.79 The introduction of alumina and titania in the formation of binary oxides ZrO2‚Al2O3 and ZrO2‚TiO234 led to the isolation of amorphous solids of large specific surface area and homogeneous composition that display a notable reduction of zirconium basicity. Using the hydrolysis of COS as a means of basicity measurement, a linear acidity increase was observed in the case of ZrO2‚Al2O3. On the other hand, the maximum acidity was reached when the composition of the mixture was 23% (mass/mass) ZrO2 and 77% TiO2. The effect of various metal additives on alumina favored iron as the metal with the highest promotion effect on the activity of COS hydrolysis.79 It appears that the hydrogen sulfide formed hastened the transformation to the corresponding sulfide, thus increasing the activity. Hydrolysis in the Presence of Tertiary Alkanolamine Solutions Littel et al.80 and Alper et al.81 studied the reaction between carbonyl sulfide and various tertiary alkanolamines in aqueous solutions in a vigorously stirred batch reactor. In the first case, the alkanolamines used were triethanolamine (TEA), dimethylmonoethanol amine (DMMEA), diethylmonoethanolamine (DEMEA), and methyldiethanol amine (MDEA). The reactions using the first three compounds were studied at 30 °C, whereas the reaction of MDEA was studied in the temperature range 20-50 °C.80 In Alper’s work,81 the reaction kinetics was studied with 0.1-1.5 kmol/m3 alcohol solutions of primary (monoethanol amine, MEA; diglycolamine, DGA; and 2-amino-2-methyl-1-propanol, AMP) and secondary (diethanolamine, DEA; 2-(methylamino)ethanol, MAE; di2-propanolamine, DIPA; and morpholine) amines. The solvents used were methanol, 2-propanol, ethylene glycol, and propylene glycol, and the experimental results were obtained via a conductometric stopped-flow technique at 5-25 °C. The kinetic data indicate apparent orders for the amines between 1 and 2, which suggests that the formed thiocarbamate involves a zwitterion intermediate80-84 in the following mechanism
COS + R2NH T R2NH+ COS-
(9)
R2NH + COS- + B f R2NCOS- + BH +
(10)
where B is any base present, such as the amine or water solvent. This contradicts Sharma’s postulate of thiocarbamate single-step formation.85 Alcohol solvents, and especially glycols, seemed to increase the reaction rates. Lee et al.80 used a wetted sphere absorber to follow the kinetics of the reaction between COS and aqueous methanolamine (MEA) over the temperature range of 25-75 °C (298-348 K) and the amine concentration range of 5-20% (mass/mass). Their kinetic data also supported the zwitterion mechanism, and they concluded that the overall reaction rate is first-order in COS and second-order in MEA concentration.86 A two-step reaction mechanism was proposed to justify all of these phenomena that is regarded as the base-catalyzed analogue of the reaction mechanism proposed earlier for the polarographic hydrolysis of COS23
COS + R3N + H2O T HCO2S- + R3NH +
(slow) (11)
HCO2S- + R3N + H2O T HCO3- + R3NH + + HS(fast) (12) The second (slow) step was found to be the ratedetermining step under ambient conditions. The fitted forward reaction rate constants for the first step of the proposed mechanism were explained by a Bronsted-type relationship that makes the reaction rate constant both temperature- and (ethanolamine-) basicity-dependent. From the ratio of the forward and reverse reactions of the slow step (step 1), a value of the equilibrium constant could be calculated that was found to be independent of the nature of the amine. As in the case of isocyanates (R-NdCdO) hydrolysis,87-89 tertiary amines appear to catalyze the hydrolysis process.23,90-93 In the case of isocyanates, Baker and Holdsworth94 suggested the formation of a slow complexation between the isocyanate (eq 13) and the amine B that quickly leads to the destruction of the complex by the nucleophilic alcohol (eq 14)
R-N)C)O + B T complex
(13)
complex + R′OH f RNHC(O)R′ + B
(14)
The rate constant of the reaction appears to be strongly influenced by the catalyst structure.87,88 Thus, a sterically hindered substituted aniline exhibited no activity, whereas the unhindered tertiary amine 1,4-diaza[2.2.2]bicyclo octane (DABCO) showed an enhanced activity.23 Ernst and Chen95 investigated the effect of DABCO and other related compounds on the hydrolysis of COS and concluded that the kinetics were first-order with respect to dissolved COS and DABCO concentration. Comparable rate constant data were obtained in both a batch reactor95 and a continuous flow reactor.23 The basicity of the amine as well as steric effects appear to play a role in determining the reaction rate, as shown by a Bronsted plot over the pKb range 10-14, and a similarity with the hydrolysis of isocyanates was established.21 6. Effect of COS in Bulk Hydrocarbons During the processing of natural gas and petroleum, various natural gas liquids (NGLs) and liquefied petroleum gases (LPGs), such as ethane, propane, and butane, are produced. The exact compositions of these
Ind. Eng. Chem. Res., Vol. 41, No. 22, 2002 5327 Table 4. Approximate Composition of the Total Percent Sulfur Impurities Contained in the Hydrocarbon Fractions Isolated during the Processing of Natural Gas hydrocarbon fraction
H2S (%)
COS (%)
mercaptans (%)
ethane propane butane
50 50 -
10 90 -
90 10
fuel gas liquids vary greatly from source to source, and the compositions of any final commercial products will vary as well. In the case of LPG, the specifications are set on the basis of the products of the overall product thermophysical properties and especially calorific value; thus, composition is often a secondary consideration. The main exception to this occurs when water or sour (acid) gases are present. Treatment of sour gas feeds is necessary to remove undesirable sulfur-bearing impurities that include hydrogen sulfide, carbonyl sulfide, and low-molecular-weight mercaptans. Table 4 displays the approximate relative percent composition of sulfur impurities in the hydrocarbon fraction. Thus, removal of these compounds, which can frequently corrode the copper and brass tubing used in propane-fed appliances, is important. Although not a sulfur compound, carbon dioxide is usually considered as an acid gas constituent as well. The corrosive nature of the acid sulfur gases in fuel gases such as LPG is reflected in the failure of the ASTM copper strip corrosion test.96,97 In this test, a strip of cleaned, polished copper is suspended in a vessel that has been rinsed with water and pressurized with an appropriate quantity of LPG. The filled vessel is then maintained isothermal at 38 °C (100 °F) for 1 h, after which the strip is removed and immediately “read”. The reading of a copper strip is done by comparison with lithographed standard strips provided by ASTM. The lithographs are divided into five regions. First, a pristine, freshly polished strip is provided. This pristine strip does not have a rating or classification beyond “freshly polished”. Next, four subdivided levels of progressive corrosion are presented: level 1 (with 1a and 1b slight tarnish), level 2 (with 2a-2e moderate tarnish), level 3 (with 3a and 3b dark tarnish) and level 4 (with 4a-4c severe corrosion). For LPG samples, 1a and 1b are considered passing, but anything higher is considered failing. Engineering literature indicates that a COS concentration of 58 ppm (mass/mass) will cause failure of the copper strip corrosion test, with the presumed mechanism being hydrolysis of COS to H2S,98,99 although recent work on pure COS100 and anecdotal descriptions of industrial experience place the threshold concentration at a much higher level. A synergetic effect is often observed in industrial samples in which traces of various sulfur compounds can combine to produce a failed test, even though the individual components will not produce failure.101,102 In this respect, it appears that the total sulfur content is less important than the individual sulfur species present. This issue will be described in more detail later. Vapor and liquid equilibrium phase composition studies of propane and carbonyl sulfide have been done using refractive index measurements in the temperature range from -7 to 80 °C (266-355 K). No azeotrope appears to form throughout this temperature range. The equilibrium ratios were calculated for each component at each temperature from the phase composition data. Equilibrium phase densities were calculated via the Lorentz-Lorenz molar refractivity relationship.103
Table 5. Pseudo-First-Order Rate Constants and Half-Lives for COS Hydrolysis in Moist Hexane in the Presence of Base base (25 µL, 1.0 M)
rate constant (min-1)
half-life (min)
water diethanolamine (1.0 M) diethanolamine (0.5 M)/ NaOH (0.5 M) sodium aluminate (0.8 M)/ NaOH (0.2 M) sodium hydroxide (1.0 M)
6 × 10-9
